A magnetically levitated train may be constructed with a plurality of superconducting magnets located in multiple cryostats disposed on either side of swiveled undercarriages, known as bogies, that support the train. The superconducting magnets maybe constructed of Niobium Tin (Nb.sub.3 Sn) wire because they would then be capable of operating at temperatures as high as 8K. An operating temperature above the normal boiling point of helium, 4.2K, is important because it allows the refrigeration requirement to be effectively provided by a conventional Gifford-McMahon cycle refrigerator. The latter refrigerator can produce refrigeration with reasonable efficiency at a temperature of between about 8K and 10K. Alternatively, the cooling can be provided by forced flow of supercritical helium, having a temperature below about 6K, which has better heat transfer properties and more stable flow characteristics then boiling helium.
Magnetic levitating trains are being considered which can periodically have coolant transferred to the trains from large, stationary refrigerators at service depots. In an alternative design, smaller refrigerators are located on the trains and power is transferred to these refrigerators through the guideways upon which the trains travel. Hybrid system designs are also being considered whereby large, stationary refrigerators provide a portion of the cooling and onboard refrigerators provide the balance. Irrespective of the advantages and disadvantages of each type of cooling system, the weight of the onboard cooling system is a major factor in its selection. Added weight requires larger magnetic fields and higher power requirements.
Typically, magnetically levitated trains use superconducting magnets which operate at less than about ten K and liquid cryogen as a refrigerant to cool the magnets. The prior art suggests the use of cooling systems based on the following technologies: (a) The use of NbTi wire which operates at temperatures of less than 6K to construct the coil for the superconductor magnet. Alternatively, NbSn wire is selected because it operates at temperatures of less than 9K while providing the same field strength as the NbTi wire. However, the NbSn wire is harder to fabricate than the NbTi wire. (b) The coil of the superconductor magnet is cooled by either immersion in liquid or gaseous helium or by forcing liquid or gaseous helium to flow through the wire tubing forming the coil. (c) The cooling system for the magnet is usually provided by a coolant supplied 1) from a refrigerator that is on each car of the train and which receives power from the guideway on which the train travels or 2) from a stationary, central refrigerator that transfers cooling, i.e., low temperature cryogens, to dewars (double walled containers with vacuum between the walls) which contain the superconductor magnets and are carried aboard the trains. The latter type of cooling system includes boil off gas storage facilities on board the train for periodic recovery of the gas.
Studies have been carried out on these different cooling options. Nb.sub.3 Sn wire magnets were found to be beneficial because the cooling system was lighter in weight and thereby offset the higher cost associated with the added difficulty in constructing this type of magnet. A heavier cooling system operates by simply cooling the magnets with liquid helium that is permitted to warm from about 4K to 8K. The added weight is primarily attributed to a helium recovery system which receives the boil off helium, compresses the helium and stores it in storage bottles at some higher temperature. The weight of the helium recovery system makes this option heavier than that of simply using a closed cycle refrigerator with or without supplemental liquid cryogen to provide the desired cooling.
Several different cooling systems using liquid helium in a sealed dewar are also known. In one system, the superconducting magnet coils are located in sealed dewars that are designed to withstand a pressure of about 1.8 Mega Pascals (MPa), the pressure in a dewar which has been filled with liquid helium at 0.1 MPa and a temperature of 4.2K and then warmed to 8K. The dewars used in magnetic levitating trains have a flat shape so that they can be mounted within the space provided for them by the construction of the trains. The flat shape of these dewars would require that their sides be constructed of a heavier gauge metal to prevent bending under high operating pressure and they would, accordingly, be heavier in weight then a similar dewar having a round shape. Using a system with a heavy weight is a deficiency because more energy and therefore higher costs are associated with operating the trains.